The present invention generally relates to fabrication of semiconductor devices and more particularly to a process for growing a group III-V compound semiconductor layer on a Si substrate.
Gallium arsenide (GaAs) is a typical III-V compound semiconductor material used for laser diodes and various fast speed semiconductor devices such as metal-semiconductor field effect transistor (MESFET), high electron mobility transistor (HEMT), heterojunction bipolar transistor (HBT) and the like because of its characteristic band structure and high electron mobility. Such a semiconductor device is constructed on a GaAs wafer sliced from a GaAs ingot grown as a single crystal or on a GaAs substrate grown epitaxially on a surface of a Si wafer. In the latter construction, one can avoid the difficulty of handling heavy and brittle GaAs wafer during the fabrication process of the device by using a light and strong Si wafer fabricated by a well established process. Further, one can easily obtain a large diameter wafer in such a construction. As a result, one can handle the wafer easily and reduce the fabrication cost of the device. Further, such a wafer is suited for fabricating a so called optoelectronic integrated circuits (OEIC) wherein semiconductor optical devices such as GaAs laser diode are assembled together with Si or compound semiconductor transistors on a common semiconductor chip.
When growing GaAs on Si wafer epitaxially, however, one encounters various difficulties. Such difficulties are caused mainly due to the large difference in the lattice constant and thermal expansion between Si and GaAs. For example, the lattice constant of Si is smaller than that of GaAs by about 4% and the thermal expansion coefficient of Si is smaller than that of GaAs by about 230%. From simple calculation based upon the difference in the lattice constant, it is predicted that the GaAs substrate constructed as such contains dislocations with a density in the order of 10.sup.12 /cm.sup.2. Thus, a simple epitaxial growth of GaAs layer made directly on Si substrate is usually unsuccessful. Even if successful, such a semiconductor layer inevitably contains significant defects such that they cannot be used as the substrate for semiconductor devices.
In order to eliminate these problems and to obtain a GaAs substrate layer having a quality satisfactory for a substrate of semiconductor device, it is proposed to grow a preliminary layer of GaAs on the silicon substrate at a low temperature and deposit a primary layer of GaAs further on the preliminary layer at a higher temperature subsequently. Thereby, the primary layer serves for the substrate for carrying semiconductor devices thereon.
FIG. 1(A) shows a conventional process for growing a GaAs primary or device layer on a GaAs substrate that may be sliced from a GaAs single crystal ingot. There, the temperature of the substrate is elevated to about 650.degree. C. and the GaAs primary layer is simply grown epitaxially thereon according to the well established MBE or MOCVD process. On the other hand, because of the difference in the thermal expansion and lattice constant described previously, such a simple epitaxial growth of the GaAs layer is not applicable to the case where a Si substrate is used.
In order to achieve an epitaxial growth of a GaAs layer on a Si substrate, a process shown in FIG. 1(B) is proposed (Akiyama, M. et al, J. Cryst. Growth 77, 490-497, 1986).
In the process of FIG. 1(B), the temperature of the Si substrate is elevated at first to about 1000.degree. C. in the atmosphere containing H.sub.2 and AsH.sub.3 to remove any oxide film on the surface of the silicon substrate. Next, the temperature is decreased to 400.degree.-500.degree. C. and a preliminary layer of GaAs is grown on the surface of the silicon substrate. Typically, the preliminary layer is grown with a thickness of 5-20 nm. After the preliminary layer is thus grown, the temperature of the substrate is elevated to 600.degree.-750.degree. C. and a primary layer of GaAs is deposited on the preliminary layer with a desired thickness. During the process of deposition of the GaAs primary layer, the preliminary layer crystallizes while maintaining epitaxy to the silicon substrate. Thereby, the primary layer provided on the preliminary layer is grown also epitaxially.
In the foregoing process of FIG. 1(B), however, it has been discovered that the surface of the preliminary layer becomes rough or undulated upon elevating the temperature for the deposition of the primary layer, and the rough surface of the preliminary layer is transferred to the surface of the primary layer that is grown thereon.
FIG. 2 shows a relationship between the surface roughness of the preliminary layer and the surface roughness of the primary layer grown thereon, wherein the horizontal axis represents the surface roughness of the preliminary layer while the vertical axis represents the surface roughness of the primary layer. As can be seen from FIG. 2, the roughness of the primary layer increases generally linearly with the roughness of the preliminary layer.
It should be realized that such an irregularity in the GaAs primary layer causes a serious problem such as scattering of carriers, reduced carrier mobility, and the like, particularly in the devices such as HEMT that use the 2DEG formed at a heterojunction interface between two different semiconductor layers. The fabrication of the preliminary layer with various surface roughness values as shown in FIG. 2 will be described later with reference to the present invention.